1. Field of the Invention
The invention pertains to dielectric resonator circuits. More particularly, the invention pertains to dielectric resonator circuits comprising housings adapted to prevent cross coupling between non-adjacent resonators.
2. Background
Dielectric resonators are used in many circuits, particularly microwave circuits, for concentrating electric fields. They can be used to form filters, oscillators, triplexers, and other circuits. The higher the dielectric constant of the material out of which the resonator is formed, the smaller the space within which the electric fields are concentrated. Suitable dielectric materials for fabricating dielectric resonators are available today with dielectric constants ranging from approximately 10 to approximately 150 (relative to air). These dielectric materials generally have a magnetic constant of 1, i.e., they are transparent to magnetic fields.
Generally, as the dielectric constant of the material of the resonators increases, higher center frequencies of the given circuit can be achieved.
FIG. 1 is a perspective view of a typical dielectric resonator of the prior art. As can be seen, the resonator 10 is formed as a cylinder 12 of dielectric material with a circular, longitudinal through hole 14. Individual resonators are commonly called “pucks” in the relevant trades. While dielectric resonators have many uses, their primary use is in connection with microwaves and, particularly, in microwave communication systems and networks.
As is well known in the art, dielectric resonators and resonator filters have multiple modes of electrical fields and magnetic fields concentrated at different center frequencies. A mode is a field configuration corresponding to a resonant frequency of the system as determined by Maxwell's equations. In a dielectric resonator, the fundamental resonant mode frequency, i.e., the lowest frequency, is the transverse electric field mode, TE01δ (or TE, hereafter). The second mode is commonly termed the hybrid mode, H11δ (or H11, hereafter). The H11 mode is excited from the dielectric resonator, but a considerable amount of electric field lays outside the resonator and, therefore, is strongly affected by the cavity. The H11 mode is the result of an interaction of the dielectric resonator and the cavity within which it is positioned. The H11 mode field is orthogonal to the TE mode field. There also are additional higher modes.
Typically, it is the fundamental TE mode that is the desired mode of the circuit or system into which the resonator is incorporated. However, other modes, and particularly the H11 mode, often are used in the proper circumstances, such as dual mode filters. Typically, all of the modes other than the mode of interest, e.g., the TE mode, are undesired and constitute interference.
FIG. 2 is a perspective view of a dielectric resonator filter 20 of the prior art employing a plurality of dielectric resonators 10a, 10b, 10c, 10d. The resonators 10a, 10b, 10c, 10d are arranged in the cavity 22 of a conductive enclosure 24. The conductive enclosure 24 typically is rectangular, as shown in FIG. 2. Microwave energy is introduced into the cavity via an input coupler 28 coupled to a cable, such as a coaxial cable. The energy may then be coupled to a first resonator (such as resonator 10a) using a coupling loop.
The high dielectric constant of the material out of which the resonators are formed concentrates the electrical fields within the resonators. However, most dielectric resonators have a magnetic constant of 1, i.e., they are transparent to the magnetic fields. Accordingly, the magnetic fields exist mostly outside of the resonator bodies. The electromagnetic coupling between the resonators that occurs in multi resonator circuits such as illustrated in FIG. 2 is magnetic field coupling. As is well known, the magnetic fields are orthogonal to their associated electrical fields.
Conductive separating walls 32 separate the resonators from each other and block (partially or wholly) magnetic field coupling between physically adjacent resonators 10a, 10b, 10c, 10d. Particularly, irises 30a, 30b, 30c in walls 32a, 32b, 32c, 32d control the coupling between adjacent resonators 10a, 10b, 10c, 10d. Conductive walls without irises generally prevent any coupling between the resonators separated by the walls, while walls with irises allow some coupling between these resonators. Specifically, conductive material within the electric field of a resonator essentially absorbs the ohmic component of the field coincident with the material and turns it into a current in the conductive material. In other words, conductive materials within the electric fields cause losses in the circuit.
Conductive adjusting screws (not shown) in conductive contact with the enclosure may be placed in the irises to further affect the coupling of the fields between adjacent resonators and provide adjustability of the coupling between the resonators, but are not used in the example of FIG. 2. When positioned within an iris, a conductive screw partially blocks the coupling between adjacent resonators permitted by the iris between them. Inserting more of the conductive screw into the iris reduces electric coupling between the resonators while withdrawing the conductive screw from the iris increases electric coupling between the resonators.
By way of example, the field of resonator 10a couples to the field of resonator 10b through iris 30a, the field of resonator 10b further couples to the field of resonator 10c through iris 30b, and the field of resonator 10c further couples to the field of resonator 10d through iris 30c. 
Wall 32a, which does not have an iris or a cross-coupler, entirely prevents the field of resonator 10a from coupling with the physically adjacent resonator 10d on the other side of the wall 32a. Furthermore, resonator 10a does not appreciably couple with resonator 10c and resonator 10b does not appreciably couple with resonator l0d because of 1) the various blocking walls 32a, 32b, 32c, 32d and 2) the significant distance between the resonators that the field lines would have to traverse in order to get around those walls to couple with each other.
One or more metal plates 42 may be positioned adjacent each resonator to affect the field of the resonator to set the center frequency of the filter. Particularly, plate 42 may be mounted on a screw 44 passing through a top surface (not shown) of the enclosure 24. The screw 44 may be rotated to vary the spacing between the plate 42 and the resonator 10a, 10b, 10c, or 10d to adjust the center frequency of the resonator. A coupling loop connected to an output coupler 40 is positioned adjacent the last resonator 10d to couple the microwave energy out of the filter 20. Signals also may be coupled into and out of a dielectric resonator circuit by other methods, such as microstrips positioned on the bottom surface 44 of the enclosure 24 adjacent the resonators.
The sizes of the resonators 10a, 10b, 10c, 10d, their relative spacing, the number of resonators, the size of the cavity 22, the size of the irises 30a, 30b, 30c, and the size and position of the metal plates 42 all need to be precisely controlled to set the desired center frequency of the filter, the bandwidth of the filter, and the rejection in the stop band of the filter. More specifically, the bandwidth of the filter is controlled primarily by the amount of coupling of the electric and magnetic fields between the resonators. Generally, the closer the resonators are to each other, the more coupling between them and the wider the bandwidth of the filter. On the other hand, the center frequency of the filter is controlled in large part by the size of the resonator and the size and the spacing of the metal plates 42 from the corresponding resonators 10a, 10b, 10c, or 10d. 
Thus, while the presence of separating walls such as walls 32a, 32b, 32c, 32d may be desirable in order to control the coupling between the adjacent resonators to the desired level, they generally lower the quality factor Q of the circuit. Q essentially is an efficiency rating of the system and, more particularly, is the ratio of stored energy to lost energy in the system. Parts of the fields generated by the resonators pass through all of the conductive components of the system, such as the enclosure, separating walls tuning plates, and adjusting screws and inherently generate currents in those conductive elements. Those currents essentially comprise energy that is lost to the system.
Occasionally, controlled cross coupling between non-adjacent resonators is desirable and can be provided by the incorporation of cross coupling mechanisms. For instance, U.S. Pat. No. 7,057,480 issued Jun. 6, 2006, which is incorporated fully herein by reference, discloses various mechanisms for cross-coupling a non-adjacent resonators in a resonator circuit.
However, in the majority of dielectric resonators filters and other circuits, cross coupling between non-electrically adjacent resonators is not desired.
Therefore, it is an object of the present invention to provide an improved dielectric resonator circuit.
It is a further object of the present invention to provide a dielectric resonator circuit having improved coupling isolation between non-adjacent resonators.